1. Field of the Invention
The present invention pertains to measuring low frequency electromagnetic fields at earthen surface, either on land or at the floor beneath a body of water.
2. Description of the Related Art
Electromagnetic (EM) soundings probe the electrical conductivity as a function of depth in the ground. Typical targets of interest are hydrocarbons, water, and ore bodies. Since the conductivities of such targets and the surrounding medium are generally quite dissimilar; they can, in theory, be discriminated by means of measurement of the subsurface conductivity. Using this methodology, the depth, thickness and lateral extent of materials of interest can be determined, depending on the availability of naturally produced low frequency EM waves or an EM source with the appropriate configuration and power.
A number of measurement scenarios are employed, including electric and/or magnetic sources, surface methods with many different source receiver geometries, borehole-to-surface methods, and cross borehole measurements. The principal passive sounding method is the magnetotelluric (MT) technique, in which the electric and magnetic amplitudes of long-period waves from natural planetary EM sources are monitored near the surface in order to determine the subsurface electrical impedance as a function of wave skin depth. Active methods include both spectral and time domain measurements of the fields in response to artificially generated waves. In the time domain, the decay of secondary magnetic fields generated from subsurface currents in response to an EM pulse under operator control can be recorded to estimate subsurface conductivity. Specific arrangements of sensors can be used to tailor the sensitivity to target subsurface features.
A common factor in electromagnetic soundings is the need to emplace and move sensors. For an electric field, the local electric potential is measured in two locations by electrically conducting electrodes buried near the ground surface. The difference between these measurements divided by the separation distance between the electrodes gives the electric field along the line of separation. For a magnetic field, a single sensor is placed upon the ground or buried at a shallow depth. Generally, it is desired to record the electric and magnetic fields in multiple orthogonal axes. For surveys on land, individual sensors are emplaced separately. As a result, so far as is known, the time taken to deploy the sensors and ensure they are oriented in the desired directions and with the desired orthogonally can be significant.
U.S. Pat. No. 5,770,945 related to a seafloor magnetotelluric system for measurement of electromagnetic fields underwater having two electric field sensor axes and two magnetic field sensor axes. Systems of this type weighed several hundred kilograms (kg) in air and were deployed and retrieved via a crane on a boat. Another sensing system (U.S. Published Patent Application No. 2008/0246485) measured three components of both the electric and magnetic field. However, this type of system still weighed in excess of 100 kg, making its use on land a significant operational challenge.
In some cases it has been desirable to collect electromagnetic data both on land and in adjacent locations underwater. Given the effort necessary to mobilize sensors to areas of use, many of which are remote, it would be desirable to have a dual mode sensor system that can work on land and underwater. In addition, it would only be necessary for a survey provider to purchase one dual mode set of sensor equipment rather than two individual sets that can work only in one environment.
In the prior art, the technologies to make electric field measurements on land and underwater were entirely different. Sea water provided a very favorable medium for coupling to electrical potentials, having high conductivity and a ready supply of reactive chloride ions that exchanged charge with standard metal salt electrodes.
On land, prior geophysical electrodes fell into two categories depending on the frequency of operation. Above 1 Hz solid metal electrodes (stainless steel, phosphor bronze) were generally hammered into the ground. Below 1 Hz, metal/metal salt combination electrodes (Ag/AgCl, Cu/CuSO4, Pb/PbCl2) were buried in excavated holes. The metal electrode was encased in a pot filled with wet mud (e.g., bentonite) that contained the required ions (Ag, Cu, Pb, and Cl). For improved performance, the pot was buried in a hole backfilled by the original ground material mixed with electrolyte. The pot coupled to the prepared ground, by means of the salt solution slowly leaking into the surrounding environment through a porous section of the pot. For convenience, both solid metal and metal/metal salt electrodes are for the purposes of the present invention termed salt electrodes. They operated based on electrochemical principles and relied on an exchange of ions with the ground in order to transfer electric charge, and thereby measure the local electric potential.
In contrast, magnetic field sensors used in geophysics on land and underwater could be the same for both purposes. However, electric field sensors generally provided the bulk of geophysical measurements, with arrays of up to 1000 sensors being used, while as few as two magnetic field sensors were needed. Accordingly, the significant difference in the configuration of the electrodes used on land and underwater has up until now precluded a single E-field sensor in particular, and a single electromagnetic sensor unit in general, from being used in both environments.
Capacitive electric field systems existed that coupled to local electric potentials via electromagnetic rather than electrochemical coupling. Capacitive sensors have the capability to operate both on land and underwater. However, the electrical contact impedance when coupling to water and dry ground are quite different. As a result, so far as is known, present capacitive E-field measurement systems are adapted to one type of environment (land or water) but did not operate adequately in both.